1. Field of the Invention
The invention refers to a circuit for changing a periodic intelligence signal with overlaid disturbance signals into a direct-current signal the amplitude of which corresponds to that of the intelligence signal, the circuit including
an input amplifier, PA1 a second amplifier behind the input amplifier, PA1 two sample and hold circuits controlled in such a manner that one of them takes over and holds the input signal of the second amplifier at each positive half-wave of the intelligence signal, and that the other takes over and holds the output signal of the second amplifier at each negative half-wave of the information signal, PA1 and a sum- and difference amplifier for receiving the signals of the sample and hold circuits via its inputs and for generating the direct-current signal.
2. Description of the Related Art
There are cases when an electric intelligence signal has the form of a rectangular pulse train of known frequency and phase position, in which cases the physical information to be evaluated determines the amplitude of this rectangular signal (intelligence signal). In magnetically inductive flow meters, for example, solenoids are activated by pulsed direct current. The fluid the flow of which is to be measured flows through the magnetic field of the solenoids. If this fluid is conductive, a potential difference is generated at the electrodes touching the fluid, the potential difference being proportional to the quantity of flow per time unit. It is known to activate the magnets of a flow determinator by pulsed direct current (U.S. Pat. No. 4 157 035). The intelligence signal received at the electrodes is applied to two sample and hold circuits that perform a sampling in different phases of the intelligence signal respectively. The output signals of the sample and hold circuits are transmitted to both inputs of a sum- and difference amplifier generating the direct current signal out of these signals.
The amplitude of the information signal often amounts to a few .mu.Volts only. In an industrial area with many electrical disturbances it is generally difficult to evaluate small voltages of this kind. In the case of flow meters, additional disturbance signals are effected for further reasons related to hydraulic aspects and possibly leading to jumps in potential at the electrodes. Moreover, the evaluation of signals is impeded by the impulse signals having a comparatively low frequency and the signal source for generating the intelligence signal has internal resistance which varies in dependence of the fluid conductivity. Furthermore, the mentioned circuits are internally disturbed by inductive voltage peaks being induced into the electrode loop during the switching phase of the exciting current.
As to the known circuits, different measures are taken for eliminating the disturbance signals: In order to protect the evaluating circuit from a direct-current signal occuring at the electrodes, the electrodes are capacity-coupled to the evaluating circuit. Transient processes of the magnetic current result in inductive pulse pikes and eddy current effects. The influence of these disturbing signals is eliminated by waiting for the completion of the activating processes after the switching of the magnetic current and only then taking over the signal voltage into one of the sample and hold circuits. The influence of stationary disturbances from the power network can be suppressed by selecting an appropriate pulse frequency of the intelligence signal and by filter circuits. For suppressing momentary disturbances, also filters can be used. A further problem is posed by the amplifier drift which can be widely eliminated by a feedback circuit.